I could be wrong but I doubt 50 mill would be enough for pollo to buy his way out of any deals with the cartel, and the kartel already knows Walt and Jesse killed that drug lord guy which is why they sent those hitmen last season. Pollo is probably thinking Walter is worth way more alive than dead which is why he is sticking his neck out protecting his investment.
There might be more to it that has not been explained yet but 50 million is just not enough to buy off the cartel when they probably make more than that a year with their deal
Ok, but think on this. The two hitmen were able to find out about Walter White easily from talking with puco's Uncle Tio. Seems like to me they would be able to get to Walter again easily if they wanted too. Why go through the trouble of daylight hits on Chicken trucks when you could just pop Walter anytime he's out of the lab. Also I think that the two hitmen were on some personal shit cause they were Tuco's cousins and wanted revenge personally.
The Carterl is about biz. They want the product or more specifically the manufacturing process. At the moment only Walt can do it, but they can find another Gayle somewhere on the planet if need be